The optical and radio counterpart of Circinus X-1 (3U 1516-56).

Circinus X-1 (3U1516-56) has a radio counterpart which, at high frequencies, shows flares with the same 16.6-day periodicity as the X-ray intensity. In each cycle the radio flare occurs shortly after the intensity drop-off which defines the X-ray modulation. The radio source is positionally coincident with a faint red star having very strong H-alpha and weak He I emission lines which are probably variable. The object may be an early-type emission-line star or a symbiotic star at a distance of 10 kpc.